Collaborative Research: Testing the Role of Magma and Related Fluids in Early-Stage Rifting, East Africa

该项目的目标是通过针对坦桑尼亚北部和肯尼亚南部的东非裂谷系统的一部分，开发一种综合方法来了解在厚厚的大陆岩石圈中形成的早期和年轻的裂谷带。这些裂谷的发展是一个基本的板块构造过程，但裂谷形成和早期演化背后的驱动力却知之甚少，特别是岩浆和构造过程（岩脉侵入与断裂）之间的应变调节的反馈和权衡。此外，热液流体和先前存在的构造在削弱岩石圈和聚焦变形和岩浆活动中的综合作用在裂谷系统发育中几乎没有被探索。本项目旨在解释岩浆系统在早期裂谷下的相对作用，大型边界断裂和分布在裂谷上的正断层系统的发育，以及热液流体在弱化厚大陆岩石圈和促进集中裂谷带发育演化中的地球化学影响。该分析将利用：(1)断层系统几何和动力学的基于现场的映射来表征与岩浆系统相关的应变空间模式；(2)利用地球物理成像技术（宽带地震阵列）对岩石圈内的岩浆和断层系统以及沿裂谷带段长度方向的地壳和上地幔结构进行三维可视化；(3)对沿边界断层系统的热液泉的气体排放进行采样，以确定可能导致岩石圈内减弱的流体的化学特征；(4)对断裂系统暴露的火山产物进行地质年代学分析，为断裂事件与岩浆事件的关系提供时间约束，并允许对时间平均应变率进行量化。这项研究将有助于建立裂谷早期岩浆和断裂系统相互作用的综合模型，为评估全球大陆裂谷演化提供一个基准。大陆裂谷带是一个谜，因为它们在厚而强的大陆岩石圈中发育，原因尚不清楚。它们的发育是板块构造过程的关键，促进了大陆的分裂，在洋中脊扩张的过程中形成海洋盆地。世界范围内的裂谷带通常与大震级地震和火山活动有关，要么集中在离散的火山上，要么从数十公里长的裂缝中喷发出来。因此，它们构成了地质灾害，威胁着全世界生活在裂谷带环境中的人类，包括美国。过去对裂谷带发展的高级阶段（在大陆分裂之前）的研究表明，裂谷作用使大陆岩石圈不断伸展和变薄，从而使地幔在裂谷下涌出并融化。然而，在年轻和发育中的裂谷系统中，岩浆似乎对裂谷作用很重要，尽管在那个阶段很少有地幔上涌。这表明，尽管存在厚厚的大陆岩石圈，岩浆仍然能够形成并上升到地壳中，从而使地壳伸展并帮助推动断裂。岩石圈显然在某种程度上被削弱了，使得岩浆活动如此集中。该研究小组假设，岩石圈的弱化是由与岩浆系统发展相关的集中的深部地幔流体协助的，这些流体以化学方式改变和削弱岩石圈，使岩浆侵入并推动裂谷的发展。在东非裂谷的这项工作将使研究人员能够确定这些流体和侵入岩浆在削弱岩石圈和允许断层发育方面的相对作用。通过结合实地考察、地球物理学、地球化学和地质年代学，将制定一个跨学科的裂谷发育和演化模型，并将捕捉裂谷过程的多个方面。这项工作的结果将适用于全球裂谷带的发展，因此可以整合到现有的板块构造模型中，为为什么大陆分裂是可能的提供一个基本原理。

The goal of the project is to develop an integrative approach to understanding incipient and youthful rift zones developing in thick continental lithosphere by targeting a portion of the East African Rift system in northern Tanzania and southern Kenya. The development of these rifts is a fundamental plate tectonic process yet the driving forces behind rift initiation and early evolution are tenuously understood, particularly the feedbacks and trade-offs between strain accommodation by magmatic and tectonic processes (dike intrusion vs. faulting). Furthermore, the integrative roles of hydrothermal fluids and preexisting structure in weakening the lithosphere and focusing deformation and magmatic activity are virtually unexplored in developing rift systems. This project seeks to explain the relative roles of magma systems below incipient rifts, the development of large border faults and systems of distributed normal faults across rift valleys, and the geochemical influences of hydrothermal fluids in weakening thick continental lithosphere and promoting the development and evolution of focused rift zones. This analysis will utilize: (1) field-based mapping of fault system geometries and dynamics to characterize spatial patterns of strain in association with magmatic systems; (2) 3D visualization of magmatic and fault systems in the lithosphere, and crustal and upper mantle structure along the length of the rift zone segments, using geophysical imaging (a broadband seismic array); (3) sampling of gas emissions from hydrothermal springs along border fault systems to determine chemical signatures of fluids that may contribute to weakening within the lithosphere; and (4) sampling of volcanic products exposed by fault systems for geochronologic analysis, providing temporal constraints on faulting events in relation to magmatic episodes and allowing a quantification of time-averaged strain rates. This study will result in the development of an integrated model of magmatic and fault system interactions during the early stages of rifting that can serve as a benchmark for evaluating continental rift evolution globally.Continental rift zones are enigmatic in that they develop in thick, strong continental lithosphere for reasons that are poorly understood. Their development is a linchpin of the of plate tectonic process, however, facilitating continents breaking apart, forming ocean basins by the process of mid-ocean ridge spreading. Rift zones worldwide are commonly associated with large magnitude earthquakes as well as volcanic activity, either focused at discrete volcanoes or erupting from tens of kilometer long fissures. They thus present geologic hazards that threaten humans living in rift zone environments worldwide, including the U.S.A. Past work on rift zones in advanced stages of development (prior to continental breakup) has shown that the progressive stretching and thinning of the continental lithosphere by rifting allows the mantle to well up beneath the rift and melt. In young and developing rift systems, however, magma appears to be important for rifting despite very little mantle upwelling at that stage. This suggests that, despite the presence of thick continental lithosphere, magma is able to form and ascend into the crust, enabling extension and helping to drive the faulting. The lithosphere is clearly weakened in some manner to allow magmatic activity to be so focused. This research team hypothesizes that lithosphere weakening is assisted by focused, deep mantle fluids associated with developing magmatic systems and that these fluids chemically alter and weaken the lithosphere, allowing magma to intrude and drive rift development. This work in the East African Rift will allow researchers to determine the relative roles of these fluids and intruding magma in weakening the lithosphere and allowing faults to develop. By combining fieldwork, geophysics, geochemistry, and geochronology, a model for rift development and evolution will be formulated that is integrated across disciplines and will capture multiple facets of the rifting processes. The results of the work will be applicable to rift zone development globally and can thus be integrated into existing plate tectonic models to provide a rationale for why continental breakup is possible.